1. Field of the Invention
The present invention relates to systems for the stabilization of a freely rotatable platform and, more particularly, to a system for applying gyroscopic inertia and precession to control the orientation of such a platform.
2. Description of the Related Art
A 19th-Century French scientist, J.B.L. Foucault, is credited with having given the name gyroscope to a wheel, or rotor, mounted in gimbal rings; that is, a set of rings that permit it to turn freely in any direction. Shortly after the middle of the last century, he conducted an experiment using a rotor mounted in three supporting frames (a "three-frame gyroscope") and demonstrated that the spinning wheel maintained its original orientation in space regardless of the Earth's rotation. He derived the word "gyroscope" from the Greek gyros, "rotation", and skopein, "to view"; a method of "viewing" or demonstrating the rotation of the Earth.
In this century, gyroscopes and devices operating on gyroscopic principles have come into wide use in various types of guidance systems: gyrocompasses for ships, automatic pilot systems for ships and aircraft, guidance systems in torpedoes, ballistic missiles, and the like. So-called two-frame gyroscopes, having a spinning disk gimballed for rotation about only two axes, have been used in conjunction with three-frame gyroscopes in ballistic missiles to correct turn and pitch motion. During the first quarter of this century, such two-frame gyroscopes were utilized in gyrostabilizer systems to reduce the roll of ships.
The common schoolboy's "gyroscope", consisting of a rotatable wheel and axle mounted within a ring housing which has one or more projections in line with the rotational axis for support on a pedestal or string, is commonly available as a toy and may be used to demonstrate principles of gyroscopic inertia and precession.
In any gyroscope, the property by which the spinning rotor resists rotation of its axis is known as gyroscopic inertia or rigidity in space. This gyroscopic inertia is a function of the speed of the rotor and the distribution of its mass. Rotors with a high speed and a concentration of mass toward the rim of the wheel display the strongest gyroscopic inertia. In other words, the gyroscopic inertia depends on the angular velocity and the moment of inertia of the rotor, or on its angular momentum.
In a spinning gyroscope, if an attempt is made to change the direction of the axis of rotation, the change of such direction takes place at right angles to the applied force. This is commonly demonstrated using the single-frame toy gyroscope by supporting the ring-frame of the spinning gyroscope by resting one of the axial projections on a pedestal or string at approximately right angles to the axis of rotation. Gravity exerts a downward force upon the gyroscope, but the result is rotation about the point of support in a horizontal direction. This property is called "precession" and may be defined as the tendency of the rotor's axis to move at right angles to any perpendicular force applied to it. The direction of rotation of the rotor's axis, due to precession, is a function of the direction of rotation of the rotor and the direction of the applied perpendicular force.
Newton's Third Law of Motion states that for every action there is an equal and opposite reaction. Newton's Third Law of Motion for Rotation states in effect that if object A exerts a torque on object B along a given axis, then object B exerts an equal and opposite torque on object A along the same axis.
The unrestrained or "free" three-frame gyroscope has little practical use, because its spin axis is subject to tilting and drifting owing to the rotation of the Earth. Gyro-stabilized inertial reference systems, commonly employed in Earth satellites and other space vehicles, employ sensors which are "strapped down" to the vehicle so as to provide an indication of vehicle position, velocity and attitude under all conditions for the vehicle position and motion. Such systems may also include mechanisms for providing desired corrections in vehicle orientation, in response to output signals from such sensors and the orientation of the gyroscope elements in the inertial navigation system. A few of the many examples of background art in this field may be found in U.S. Pat. Nos. 3,398,341 of Dooley et al, 3,412,239 of Seliger et al, 3,491,228 of Selvin, 3,986,092 of Tijsma et al and 4,052,654 of Kramer et al.
Satellites and other space vehicles or particular movable components thereof, such as antennas, solar cell arrays and the like, may be oriented by reference to the sun or some distant star. However, the orientation of a satellite may shift slightly over time, due to factors such as solar radiation, magnetic fields and the like. It is known to provide the necessary compensation by equipping a satellite or other vehicle with one or more spin jets which use suitably controlled and timed jets of gas carried on board the vehicle to develop the desired correction of vehicle attitude. (See, for example, U.S. Pat. Nos. 3,396,920 of Rosen et al and 3,758,051 of Williams). The problem inherent in such systems resides in the fact that the supply of gas available on board the vehicle for the purpose of attitude correction is limited and eventually the supply is exhausted, at which time the satellite or other vehicle can no longer be maintained in the desired attitude and is for all practical purposes lost.
It would be desirable to be able to control the attitude of the satellite or other space vehicle through the use of some electromechanical or electromagnetic means. Ample electrical energy for driving such devices is available from solar arrays. However, to this point, the application of Newton's Third Law of Motion for Rotation has, to my knowledge, prevented the effective utilization of electromechanical or electromagnetic systems for space vehicle attitude orientation.